Recently, for rail vehicles using an AC overhead wire, increasing attempts have been made to find technical solutions to avoid the need for the conventional network transformer. In particular, network transformers have the disadvantages that they are heavy and they incur relatively high energy losses.
One solution is to use superconducting transformers although, inter alia, they are more problematic for vehicle use owing to the cooling systems that are required, than for stationary systems, and are therefore not yet technically proven at the moment.
Other known solutions require the direct connection of the power-electronic converters to the high voltage of the AC overhead wire. Until now, the disadvantages here have been the high technical complexity and a range of restrictions which prevent universal use, as will be explained in the following text.
In general, modern electric locomotives use power-electronic converters for feeding the traction motors and the auxiliary systems (air-conditioning system and other modes). The overhead wire voltages which have been introduced throughout the world for AC locomotives have, however, been chosen to be very high. This is done in order to minimize the transmission losses (in Europe: 15 KV/16 ⅔ Hz, as well as 25 KV/50 Hz). While avoiding the need for a network transformer by being connected directly to the overhead wire voltage, these high voltages require semiconductors or converter elements to be connected in series in the power electronics—as well as a high degree of technical complexity overall.
The following variants with direct connection of the power-electronic converters to the overhead wire voltage are known:    a) Use of high-voltage insulated traction motors without DC isolation between the motors of the overhead wire, for example, is known from Steiner et al, A New Transformerless Topology for AC-Fed Traction Vehicles using Multi-Star Induction Motors, EPE 1999, Lausanne. Solutions such as these have the advantage of a small number of power-electronic converter stages in the energy flowpath between the overhead wire and traction motors. This reduces the energy losses and the complexity of the power electronics. One example is shown in FIG. 6. On the other hand, the disadvantages include:    The increased motor isolation that is required has a disadvantageous effect on the physical motor size, on the motor weight and/or on the motor efficiency.    The power matching that is absolutely essential between the series-connected converter elements (when the motors are subject to different loads) requires the use of complex motor windings with a large number of connections carrying high voltages (so-called 3-star motors).    The number of traction motors cannot be varied freely. In order to limit the disadvantageous effects of the items mentioned above, as large a number of high-power motors as possible should be chosen.    The high DC voltage components between the series-connected converter elements make it harder to provide reliable isolation in actual conditions (dirt, moisture).    The feed for the auxiliary systems requires considerable additional complexity.    In order to maintain a restricted operating capability in the event of failures in the power section or in the event of insulation faults in the motors, additional switching devices are required (redundancy).    b) Use of conventional traction motors (with a low load on the insulation) without DC isolation from the overhead wire close to ground potential, for example as known from DE 197 21 450 C1. The disadvantages and restrictions of the first-mentioned solution are avoided with this embodiment of the motors. Further advantages are good efficiency and low harmonic current levels in the railroad network. However, the voltage transformation ratio required from the high input voltage (up to 25 kV rated value) to the normal output voltages for feeding the traction motors is about 10:1. In principle, power-electronic converters which provide a high voltage transformation ratio without the assistance of transformers are worse, in terms of the complexity for energy stores and semiconductor switches, than converters of the same rating that have to provide only a low voltage transformation ratio. This fundamental disadvantage, in particular the size of the energy stores that are required, prevents universal use of corresponding variants.    c) DC isolation of the motors from the overhead wire using a number of individual medium-frequency transformers, which each have associated individual, series-connected converter elements, for example as known from Schibli/Rufer, Single and Three-Phase Multilevel Converters for Traction Systems 50 Hz/16 ⅔ Hz, LEI, Lausanne, pages 4.210-4.215. Variants of this solution have already been very widely investigated. FIG. 7 shows a corresponding circuit arrangement. The following items are characteristic:    On the network side, groups of converter elements are connected in series and, overall, produce a staircase voltage whose controllable maximum values must be greater than the network voltage peak values.    Each of these converter element groups has a four-quadrant controller on the network side, a first DC voltage capacitor, a medium-frequency inverter on the primary side, a medium-frequency transformer, a medium-frequency rectifier on the secondary side, and a second DC voltage capacitor. All of the converters must be designed for both energy flow directions (energy drawn from the railroad network as well as energy feedback), if it is intended to be possible to feed energy back into the railroad network.    A solution such as this is not subject to the disadvantages of the first solution that was described (high-voltage insulated traction motors). The insulation load on the traction motors can be kept low. There are no restrictions to the number or operating voltages of the traction motors. Power matching between motors that are subject to different loads can be carried out via the DC busbar (P0, N0 in FIG. 7). On the other hand, the disadvantages are as follows:    The large number of power-electronic converter stages which are located in the energy flowpath between the overhead wire and the traction motors (high degree of complexity, relatively high energy losses).    The high degree of complexity for energy stores (2 DC voltage capacitors plus any series resonant circuits) for smoothing the power pulsation at twice the network frequency.    The large number of individual medium-frequency transformers required, which in total are worse than one central transformer in terms of the weight and the space required. Splitting into a large number of individual transformers is also disadvantageous, and occupies a large amount of space, as a result of the (in total) numerous transformer connection points on the high-voltage side.    As in the first-mentioned variant, the high DC voltage components between the series-connected converter elements make it harder to provide reliable isolation in actual conditions (dirt, moisture).    The feed for auxiliary systems requires considerable additional complexity.    In order to maintain a restricted operating capability in the event of failures in the power section or insulation faults in the motors, additional switching devices are required (redundancy).    d) DC isolation of the motors from the overhead wire using a medium-frequency transformer which is fed from the overhead wire by means of a direct converter, for example as known from DE 26 14 445 C2 or Östlund, Influence of the control Principle on a High-Voltage Inverter System for Reduction of Traction-Transformer Weight, EPE, Aachen 1989.
Solutions such as these likewise have the advantage of a reduced number of power-electronic converter stages located in the energy flowpath between the overhead wire and the traction motors. If the medium-frequency is sufficiently high (in the order of magnitude of about 1 KHz or more), the physical size, weight and energy losses in the medium-frequency transformer may be kept considerably lower than the corresponding disadvantages in the motors. In addition, it is possible to feed the auxiliary systems from the medium-frequency transformer efficiently and with little complexity. In general, this is advantageously done by means of a separate secondary winding on the medium-frequency transformer.
However, for several reasons, the provision of a direct converter with thyristors does not satisfy present or future requirements. The major disadvantages are:    The achievable medium frequency is restricted to a few 100 Hertz owing to the commutation times in a circuit fitted with thyristors. This frequency is not sufficient to significantly reduce the weight of the medium-frequency transformer.    The stringent interference current limit values (minimizing harmonic currents in the railroad network) for modern locomotives cannot be complied with because, in principle, the spectrum includes twice the medium frequency and other interference frequencies. Twice the medium frequency is, furthermore, far too low to be adequately damped by the inductive network impedance.    The primary winding in the medium-frequency transformer is loaded with the high peak voltage values of the overhead line voltage plus their transient overvoltage spikes. This makes it harder to provide isolation (winding isolation, air gaps, creepage paths) for the transformer.
Direct converters with power semiconductors which can be turned off (in general: IGBT transistors instead of thyristors) are known in the form of so-called matrix converters, for example from Kjaer et al, A Primary-Switched Line-Side Converter Using Zero-voltage Switching, IEEE Transactions on Industry Applications, Vol. 37, No. 6, pages 1824-1831. FIG. 8 shows the basic circuit of a matrix converter with the converter branches and the filter capacitor. The converter branches are provided in a known manner by way of bi-directional controllable electronic switches. Known implementations are:    Two thyristors (GTO thyristors) which can be switched off and are connected back-to-back in parallel. These components must have a reverse blocking capability, that is to say they must be able to block both voltage polarities (FIG. 9).    Two IGBT transistors which are connected back-to-back in parallel. These components must have a reverse blocking capability (FIG. 10).    Two IGBT transistors which are connected to back-to-back parallel-connected diodes. There is no need for components with a reverse blocking capability (FIG. 11).
However, the following disadvantages of direct converters are also associated with these embodiments. These are:    The low-frequency power pulsation (at twice the network frequency: 2fN) which occurs in single-phase AC networks and must be transmitted by the medium-frequency transformer. This has a disadvantageous influence on the physical size and efficiency of the medium-frequency transformer.    The filter capacitor which is required on the network side and can cause interference resonances in the railroad network, and which can lead to the circuit having undesirably low input impedances for higher-frequency interference currents.    In contrast to converters with a DC voltage intermediate circuit (“U converters”), the power semiconductors have no protection against high-energy network overvoltages, as provided by a capacitor on the DC voltage side. This necessitates comparatively considerable derating of the semiconductor reverse voltages.    The harmonic content of the converter voltages which are produced is very high both on the network side and on the transformer side. No suitable circuits or methods are known for producing staircase voltages with a low harmonic content (analogously to multipoint U converters) for matrix converters.
In addition, for the present matrix converter applications, it is of major importance to be able to cope with high voltages and possible malfunctions without serious consequential damage in the relatively high power range. Disadvantageous items relating to this are:    In the event of a short circuit on the AC voltage side between the circuit points N1 and N2 (see FIG. 10), extremely high discharge currents flow from the filter capacitor on the AC voltage side, which can cause destruction, owing to the extremely high mechanical forces and/or arc damage that occur.    In the event of failure of power semiconductors or a faulty drive, the discharge current, which is like a short circuit, can flow directly through the semiconductors, destroying them and their contacts.    The very small stray inductance from the filter capacitor and from the converter branches which is required for the semiconductor switches in the matrix converter conflicts increasingly with the rising voltage level (with a peak value of up to about 50 KV for a 25 KV overhead wire voltage) for a design embodiment which is mechanically resistant to short circuits and is safe in terms of isolation. Furthermore, there are major impediments to unrestricted spatial arrangement of the components.
An arrangement as shown in FIG. 11 is known, inter alia from Kjaer, loc. cit. (see FIG. 3 there). In comparison to FIG. 8, this includes the following three modifications:    The filter capacitor is split into a number of capacitors. However, the resultant capacitance is still connected in parallel with the network-side connections of the matrix converter, so that the disadvantages also still remain.    Additional damping resistors are connected in a known manner in series with the filter capacitors. This measure results in high energy losses, although it has become necessary because the filter capacitors are additionally required as snubbers for the IGBTs.    The medium-frequency side of the matrix converters is based on a three-phase design. In comparison to a single-phase design, this allows somewhat lower harmonics on the network side. However, a staircase voltage that is sufficiently low in harmonics and has a freely variable number of voltage steps is still impossible.